Method and system for coupling acoustic energy using shear waves

ABSTRACT

A system and method for coupling acoustic energy within a waveguide provides highly efficient and sensitive acoustic energy generation and detection. In particular, an ultrasound angioplasty system is described which makes use of an end-fire array of ring transducers to produce highly directionalized sound within an acoustic waveguide. The transducers can be made circularly symmetric, and may be composed of multiple segments for generating sound waves in independent x and y spatial modes within the acoustic waveguide. Each ring transducer is optimally spaced 1/2λ L  from its neighbor transducers, such that alternate transducers transduce 180-degrees out of phase, and may have their electrical end inverted for common drive, or for summing of transducer electrical outputs when the array is used as a detector. The phased array may also be used in a resonant acoustic energy system used to detect pressure variations or reflections from a substance, for example, for detecting the progress of chemical reactions, liquid level sensing, etc., imaging, or in various other ultrasound applications.

The present invention relates to a method and system for couplingacoustic energy.

BACKGROUND

An acoustic energy transmission system typically transmits sound wavesto some distant point, where mechanical energy is derived from the soundand used in an application. The sound waves can be generated using anyof a number of conventional transducers, for example, audio speakers andpiezoelectric devices. These devices are caused to vibrate back andforth to convert electrical energy to movements of air; they can alsosometimes be used in the reverse sense, to convert movements of air toelectric charge. In traditional usage, these devices are coupled to avoltage generator and they responsively transmit longitudinal wavesthrough the air, that is, the air is moved back and forth in the samedirection in which the sound waves travel.

One example of an acoustic energy transmission system is an ultrasoundangioplasty system. In this type of system, ultrasound is used to clearblocked or partially blocked human arteries. The ultrasound can begenerated by an ultrasound generator, and coupled via an encased solidwire through a catheter probe positioned within the occluded artery. Theultrasound wire causes an extendable catheter tip to vibrate, therebydisintegrating arterial plaque that the extendable member contacts. Tobest perform this task, it is necessary to have strong ultrasound wavesarrive at the catheter tip. Unfortunately, use of a solid wire makes itdifficult to efficiently couple acoustic energy into the solid wire andhave strong ultrasound arrive at the catheter tip. The solid wire isalso a relatively expensive, not easily-replaced part of the system.However, solid wires are generally used in ultrasound angioplasty, sincethe solid wires facilitate probe vibration in two or more spatialdimensions, which is desired for best clearing arterial plaque.

In general, acoustic energy transmission systems such as these sufferfrom several limitations. First, the use of a transducer to createlongitudinal sound waves typically requires that the transducer have amoving surface which is perpendicular to, and directly in the path of, awaveguide, e.g., the transducer's vibrating surface moves back and forthtoward and away from the waveguide along the transmission direction.This requirement renders it difficult to channel sound from multiplelongitudinal wave transducers into a single waveguide in a reinforcingmanner. Also, this requirement makes it difficult to generatedirectional acoustic energy, e.g., sound that travels substantially onlyin a single direction without losing substantial energy via dispersion.Many acoustic energy systems therefore generally feature undesired lossof power, caused by loss of acoustic energy through walls of thewaveguide.

It is desired in many acoustic energy systems to have as little loss aspossible through the waveguide, and to produce a very strong signal atthe distant end of a transmission path. In the example of the ultrasoundangioplasty system just given, this would enable very strong highfrequency vibrations to be produced at a catheter tip inside a humanbody, using a relatively inexpensive and efficient sound generator. Inthe case of other ultrasound systems, for example, imaging systems andvarious acoustic sensors, it is also desired to have a system thatdetects weak sound signals with heightened sensitivity.

A definite need exists for an improved acoustic energy system thatcouples acoustic power through a waveguide with relatively littlepropagation loss, and which can produce and maintain intense ultrasonicwaves throughout the waveguide. Further still, a need exists for asystem that can produce complex wave patterns. In the context of anultrasound angioplasty system, such a system would be beneficial inpermitting a catheter probe to perform complex motions, enhancing theunblocking process. The usefulness of such a system would not just belimited to ultrasound angioplasty, but rather, would have applicabilityto other fields that use acoustic energy transmission, includingmeasurement and computation systems. The present invention solves theaforementioned needs and provides further, related advantages.

SUMMARY OF THE INVENTION

The present invention provides a highly efficient method and system forcoupling acoustic energy to a waveguide using at least one shear wavetransducer. In particular, the present invention uses a shear wavetransducer that transduces shear waves propagating perpendicular to thewaveguide, towards and away from its center where a "sweet spot" iscreated in the waveguide. As a result, the waveguide need not physicallybe obstructed by a transducer, as in the case of some devices thatutilize longitudinal wave transducer. The present invention therebyfacilitates use of the transducer either as a generator or detector insystems where it is advantageous to have the acoustic transducer notsubstantially impede longitudinal waves.

For example, in one aspect of the invention, a phased-array of shearwave transducers can be used as part of an ultrasound angioplastydevice, to generate intense shear waves that are efficiently coupled tothe waveguide and transmitted as longitudinal waves to an ultrasoundcatheter. In this manner, all of the transducers independently generatelongitudinal waves that reinforce each other as they travel along thecenter axis of the waveguide.

In another aspect of the invention, the shear wave transducer andwaveguide can be used to form an acoustic detector, with the shear wavetransducer detecting longitudinal waves and responsively producing anoutput signal which matches a predetermined frequency. Using multipletransducers, a phased array can be created which is highly tuned tolongitudinal waves of a particular frequency.

A more particular aspect of the invention provides a shear wavetransducer which is a ring, or annular, transducer. The ring transducercan be coupled to an oscillation source and used to generate shear wavesand direct them radially inward to converge at a "sweet spot" of awaveguide. As a result, longitudinal waves can be generated within thewaveguide and transmitted to a distal point. By using many shear wavetransducers arranged along the waveguide to form a phased array, veryintense longitudinal waves can be generated within and transmittedefficiently by the waveguide. A shear wave transducer can, applying theprinciples of the present invention, be made of different pairs oftransducer segments which are driven by different excitation sources, tosimultaneously produce different waves within the waveguide. Forexample, this aspect of the invention can be used to create complexmotions in an extendable member of an ultrasound catheter.

The invention may be better understood by referring to the followingdetailed description, which should be read in conjunction with theaccompanying drawings. The detailed description of a particularpreferred embodiment, set out below to enable one to build and use oneparticular implementation of the invention, is not intended to limit theenumerated claims, but to serve as a particular example thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of a shear wave transducer that overlies atleast two locations around a waveguide; shear waves produced at eachlocation converge within the waveguide to form a "sweet spot," which isseen at the center of FIG. 1.

FIG. 2 is a side view of the transducer and waveguide of FIG. 1, takenalong line 2--2 of FIG. 1. FIG. 2 shows sideways particle motion ofshear waves which are propagating radially inward (as indicated by thereference arrows in FIG. 1). Convergence of the shear waves at the"sweet spot" causes intense particle motion along the center axis of thewaveguide at the "sweet spot," such that longitudinal waves aretransmitted along the waveguide. An arrow within the transducer at thetwo locations indicates a poling vector of the transducer.

FIG. 3 is an illustrative diagram of a phased array of ring transducersused to reinforce very intense axially propagating longitudinal waveswithin the waveguide.

FIG. 4 shows an alternative embodiment of the ring transducer of FIG. 3.In particular, the transducer of FIG. 4 includes two pairs of transducersegments, each pair driven by an oscillation signal ω and φ,respectively.

FIG. 5 shows an alternative, square shaped transducer, used with awaveguide which is rectangular in cross-section.

FIG. 6 is a diametrical cross-section of an ultrasound generator and anacoustic waveguide of the present invention. Ultrasound is produced byeach of ten ring-shaped shear wave transducers, such that intenselongitudinal ultrasound waves are transmitted through the acousticwaveguide, in the direction indicated by a reference arrow at the rightside of FIG. 6.

FIG. 7 is a cross-section of one ring transducer and the acousticwaveguide, taken along lines 7--7 of FIG. 6. FIG. 7 shows propagation ofshear waves in a radially inward manner, as indicated by variousreference arrows appearing in FIG. 7.

FIG. 8 shows an ultrasound detector which embodies the principles of thepresent invention. In particular, FIG. 8 shows a detector which is tunedto provide an electronic output in response to the strength of acousticenergy at the predetermined ultrasound frequency.

FIG. 9 is a schematic diagram used to explain the general parts of anultrasound angioplasty system, one application of the present invention.

FIG. 10 is an illustrative diagram of an ultrasound catheter and probeused in the system of FIG. 9.

FIG. 11 is a schematic diagram of a resonant measurement system. FIG. 11shows use of two phased arrays of ring transducers, as a sound generatorand a detector, respectively.

FIGS. 12-17 illustrate construction of a shear-wave transducer havingring geometry.

FIG. 12 shows a block of PZT material that will be cored to remove acylindrical section of PZT material (indicated in phantom); an originalpoling vector of the PZT material is indicated for purposes ofillustration, which may or may not preexist in a given PZT sample.

FIG. 13 shows the cylindrical section of FIG. 12 removed from the PZTblock, and how the cylindrical section is sliced along the height of thecylindrical shape to create multiple ring transducers; in addition, acore which will be removed to form the ring geometry is indicated inphantom.

FIG. 14 indicates deposition of metal electrodes on opposite lateralsides of a ring from FIG. 13. The electrodes are coupled to a highvoltage, to set a poling vector which is normal to the ring (i.e.,parallel to the direction of the ring's lateral thickness).

FIG. 15 shows the ring from FIG. 14, where the metal electrodes areremoved and new peripheral excitation electrodes are deposited.

FIG. 16 is a cross-sectional diagram showing simultaneous vacuum-chamberdeposition of electrodes on radially inward and outward peripheries ofmultiple ring transducers.

FIG. 17 is a cross-sectional diagram showing one ring transducer of FIG.16, installed on an acoustic waveguide (in phantom), and the variouselectrical connections associated therewith.

DETAILED DESCRIPTION

The invention summarized above and defined by the enumerated claims maybe better understood by referring to the following detailed description,which should be read in conjunction with the accompanying drawings. Thisdetailed description of several particular preferred embodiments, setout below to enable one to build and use certain implementations of theinvention, is not intended to limit the enumerated claims, but toprovide particular examples thereof. The particular examples set outbelow are the preferred implementation of devices for coupling acousticenergy, for example, an ultrasound generator and an ultrasound detector.The invention, however, may also be applied to other types of systems aswell.

I. Introduction To The Transducer Elements And End Fire Array Used InThe Preferred Embodiments

FIGS. 1-3 are used to illustrate basic principles of the presentinvention. In particular, FIGS. 1-2 show use of a shear-wave transducer21 to transduce acoustic energy without interfering with the passage oflongitudinal waves which are traveling along a waveguide 23. FIG. 3shows an end-fire array 25 of multiple transducers 27, 28, 29 whichcombine to efficiently transduce intense acoustic energy, eithergenerating acoustic energy or, alternatively, detecting it in thewaveguide.

FIG. 1 shows a general case where the shear wave transducer 21 has twodiscrete segments 31 and 32 that lie about a periphery 33 of a waveguide23. The waveguide 23 may be circular in cross-section, as seen inphantom lines of FIG. 1, or it may be any other shape. The waveguide 23has a transmission axis (or waveguide axis) 35 along which it is desiredto transmit acoustic energy, for example, ultrasound; the waveguide axis35 appears as a dot in FIG. 1, and is normal to FIG. 1, extending intoand out of the paper on which FIG. 1 is drawn.

The shear wave transducer 21 can be used either as an acoustic energygenerator, in which case electrical signals cause the transducer togenerate shear waves and direct them toward a middle area 37 of thewaveguide (as indicated by the reference arrows 39) or, alternatively,as an acoustic detector, in which case shear waves travel in the reversesense. For purposes of this introductory section, it will be assumedthat the transducer 21 is being used as an acoustic energy generator.

The transducer 21 is structured to direct identical shear waves in aconverging manner, toward a "sweet spot" 41 of the waveguide. In thisregard, "shear waves" are waves that cause particle motion perpendicularto the waves' direction of travel, like deep water ocean waves."Longitudinal waves," by contrast, cause particle motion along thedirection of travel. As seen in FIG. 1, the reference arrows 39 indicatethe direction of shear wave propagation, which is perpendicular to thedirection of particle motion (the latter occurring in a direction normalto FIG. 1, into and out of the paper upon which FIG. 1 is drawn). Inthis manner, as the shear waves converge, particle motion becomes moreintense, and is most intense at the "sweet spot" 41. While FIG. 1 showsa general case where only two discrete transducer segments 31 and 32 areused, more segments may be used, for example, around a circularwaveguide, in which case particle motion at the "sweet spot" is evenfurther enhanced. As indicated by an outer circle 43 of FIG. 1, thetransducer 21 may be made continuous around the waveguide 23, as with aring transducer, in which case particle motion will be even moreintense.

FIG. 2 shows in cross-section the transducer 21 and waveguide 23 of FIG.1, taken from a vantage point identified by line 2--2 of FIG. 1. Inparticular, the two discrete transducer segments 31 and 32 are seen tohave a poling vector 45, which indicates direction of particle motionwhen the transducer 21 is excited by an electrical signal.Back-and-forth particle motion is indicated by the various arrows 47and, as illustrated in FIG. 2, the motion becomes more intense closer tothe "sweet spot" 41. As seen in FIG. 2, the "sweet spot" 41 extendslongitudinally along the waveguide 23, approximately at the waveguide'stransmission axis 35.

FIG. 3 shows the end-fire array 25 of several shear wave ringtransducers 27, 28, 29 which are configured to either sense or generateultrasound optimally having a predetermined frequency. Configuration ofthe array 25 is also briefly introduced here in the context of anultrasound generator, before a discussion of ultrasound angioplasty andmeasurement system embodiments of the present invention. Additionaldetails of the construction of the array 25 and its use as an ultrasounddetector will also be provided further below.

Each ring transducer 27, 28, 29 has a dedicated set of electronic leads49 which supply the transducer with a sinusoidal signal 50 and cause thetransducer to responsively vibrate and generate ultrasound. Eachtransducer 27, 28, 29 is specially constructed to generate shear wavesof ultrasound which are directed radially inward, toward the center ofthe ring shape of each transducer. To this effect, each transducer ismade from specially-processed piezoelectric material (PZT) and is formedto have (1) a radial thickness of 1/2λ_(PZT) (where λ_(PZT) correspondsto the shear wave velocity V_(s) in the PZT material), (2) electrodes ofopposite polarity 51 and 52 existing on radial edges of the ringgeometry, and (3) poling which is perpendicular to the ring geometry(i.e., parallel to the axis 35). The innermost radial electrode 51 ofeach transducer is optimally used as a ground electrode, while theoutermost electrode 52 of the transducers are driven by the sinusoidalsignal. The sinusoidal signal 50 as it is imparted to the outermostelectrodes 52 is generated by an excitation source, and is described bya frequency ω and a variable phase lag φ. All of the transducers 27, 28,29 receive a proper phase lag with respect to their spacings apart, suchthat they each reinforce intense longitudinal ultrasound waves that arepropagated along the waveguide axis 35, which is a common center axis ofall of the transducers. Thus, in the preferred case where ten ringtransducers are used, intense, highly-directional longitudinal waves canbe generated along the waveguide axis 35. This configuration providesfor highly efficient acoustic coupling, particularly in applicationssuch as ultrasound angioplasty, wherein the waveguide 23 is a solidmetal wire.

Preferably, each transducer (transducer 28, for example) is spaced1/2λ_(L) from its neighbor transducers 29 and 29 (where λ_(L) dependsupon the longitudinal ultrasound velocity V_(L) in the transmissionmedia, i.e., in the waveguide material); this configuration isparticularly desirable, since 180-degree opposite phases of anoscillation signal are readily derived from a phase splitter orpush-pull driver, such as a center tap transformer. However, otherphasings and spacings between the transducers of the array 25 arepossible, as will be apparent to those of ordinary skill in the art.

The shear wave transducers do not necessarily have to be shaped ascontinuous rings. For example, FIG. 4 shows a transducer 55 having twodistinct transducer segment pairs 56 and 57, each having two opposingsegments 58. Each pair 56 and 57 receives an oscillation signal ω or φ(of different frequency) and propagates shear waves radially-inwardtoward a center 59 of the waveguide, as indicated by reference arrows60. Notably, the location of the "sweet spot" (or perhaps plural "sweetspots") for the transducer of FIG. 4 depends upon the arrangement of thepairs 56 and 57 and any relative phase lag imparted to oscillationsignal ω or φ within each pair.

FIG. 5 shows a transducer 61 that has two opposing flat segments 62 and63 which bracket a rectangular waveguide 64. In this configuration,shear waves are directed to a middle plane 65 of the waveguide, asindicated by reference arrows 67, with a planar "sweet spot" 66 beingformed throughout the middle of the waveguide.

As will be seen from this introduction therefore, an end fire array ofshear wave transducers can be used to produce highly-directionalultrasound that propagates intensely along a waveguide axis 35. Asdiscussed further below, the end fire array 25 can also be used as ahighly-sensitive, frequency specific detector, in which case theelectric leads 49 provide electronic outputs from each of thetransducers.

II. The End Fire Array Used As An Acoustic Generator

FIG. 6 provides a cross-section of the ultrasound generator 71, whichcouples sound to an acoustic waveguide 73. In particular, a first end 75of the waveguide 73 is fitted with ten ring transducers 76 which arebonded with an epoxy to a circular periphery 77 of the waveguide. Atthis first end 75, the waveguide is also coated with a conductivematerial 79 (preferably a gold-based mixture is used, although any thinfilm conductive material can be used which adheres well to thewaveguide), the conductive material being connected to a center tapconnection 81 (i.e., ground) of a transformer 83. It is this transformer83, and an oscillator 84, which together form the excitation source 82that generates the push-pull oscillation signal.

Each of the ring transducers is spaced apart from its neighbortransducers at intervals of 1/2λ_(L), the ring transducers beingseparated by Teflon spacer rings that rigidly maintain the spacingbetween adjacent transducers. The transducers are excited by oppositepower phases provided by end-taps 85 and 87 of the transformer. Theopposite power phases provided by the transformer are alternatelycoupled to outermost radial transducer electrodes 101. As a result, theten ring transducers 76 generate longitudinal waves that arehighly-directional within the acoustic waveguide in both directionsalong a transmission axis 91 of the waveguide, as indicated by arrows 92and 93. However, the first end 75 of the waveguide 73 is terminated witha polished face 89 at a distance of 1/4λ_(L) from a first one of thetransducers, such that longitudinal waves emerging from the transducerstoward the left side of FIG. 6 (as indicated by reference arrow 92) arereflected back along the transmission axis 91. These reflected waveshelp reinforce production of longitudinal waves directed toward adistant, second end of the waveguide, as indicated by the referencearrow 93 in FIG. 6.

FIG. 7 is a cross sectional view of a single ring transducer 94, takenacross lines 7--7 of FIG. 6. Several arrows 95 indicate the direction ofpropagation of shear waves generated by the ring transducer 94 towardthe center of the waveguide (i.e., the transmission axis, which appearsas a point 98 in FIG. 7). Particle movement for the shear waves occursin a direction perpendicular to FIG. 7, into and out of the drawing (andalong the transmission axis, which is designated in FIG. 6 by thereference numeral 91). Since shear waves converge at the center point98, particle movement is strongest at that point. Preferably, thediameter of the acoustic waveguide is such that the waveguide supportsonly a single mode of wave propagation, to best maintain the strength ofparticle movement.

FIG. 7 also illustrates innermost and outermost electrodes 99 and 101 ofthe transducer 94. As mentioned earlier, each transducer is composed ofa piezoelectric material which is poled in a manner to generate shearwaves. The electrodes 99 and 101 are non-conventional in the sense thatthey are added to the radial edges of the ring transducers, with theoutermost electrode 101 preferably coupling a signal having a particularphase to the transducer, and the innermost electrode 99 providing acommon ground for each transducer. Importantly, each transducer has aradial thickness of 1/2λ_(PZT) (λ_(PZT) =tV_(PZT), where V_(PZT) is theshear wave velocity in the PZT material) such that it is configured tooptimally generate waves having frequency ω=v_(PZT) /λ_(PZT) (e.g., afew centimeters) when coupled to an oscillation signal of the samefrequency. An inner bore of the transducer is made to correspond closelyto a diameter of the waveguide 73 such that, during assembly, eachtransducer may be snugly fitted over the acoustic waveguide and adheredthereto, if necessary, using a conductive adhesive.

III. The End Fire Array Used As Acoustic Detector

FIG. 8 illustrates an acoustic detector 103. In particular, the detectoralso includes an end fire array 107 composed of ten ring transducers 109which are mounted to the periphery of an acoustic waveguide 111. Eachtransducer 109 is spaced apart by 1/2λ_(L), and produces an electronicoutput on signal leads 113 which represents contribution to acousticenergy within the waveguide at a predetermined frequency ω (which isthat frequency which matches the characteristics of the end fire arrayin terms of transducer thickness, etc., as has been previouslydescribed). Longitudinal acoustic waves traveling along the waveguideare indicated by the reference arrows 115. These waves will be dampenedsomewhat near the periphery 117 of the waveguide, giving rise to shearwaves which diverge radially from the center of the waveguide and towardthe transducers, as indicated by the reference arrows 119 of FIG. 8.Vibrations are thereby imparted to the PZT material of each transducer109, causing each transducer to generate an electronic signal havingfrequency ω (where ω=v_(L) /λ_(L), v_(L) being longitudinal wavevelocity in the transmission media). Since each transducer is spacedapart by 1/2λ_(L), every other transducer will be 180-degrees out ofphase (providing output signals φ₁ and φ₂ of FIG. 8). Accordingly, eachtransducer's output signal φ₁ or φ₂ may be passed conveniently toalternate taps 119 or 121 of a center tap transformer 123, and used togenerate an array output signal 125 having frequency ω. As before, acenter tap 127 of the transformer 123 is connected to a peripheralconductor 129 of the waveguide 111 to provide a ground for alltransducers.

The array output signal 125 can be utilized in a wide variety ofapplications where it is desired to have an acoustic detector which ishighly tuned to specific frequencies, for example, in variousmeasurement systems. For example, as will be explained further below,the array output signal 125 can be coupled to electronics and a visualdisplay (not seen in FIG. 8) used to indicate to a user a characteristicof detected acoustic waves. The visual display could be used, forexample, to display distance to a detected object, pressure as itaffects a special waveguide, or molecular structure as a chemicalreaction proceeds.

IV. Application To Ultrasound Angioplasty

The preferred application of the invention is in the field of ultrasoundangioplasty. In practice, a patient's bloodstream is injected with adye, which gives rise to a strong visual contrast on a video angiogramdisplay. This display (not shown in the accompanying figures) relies onx-ray fluoroscopy to display and highlight the occluded blood vesselsegment, blood vessel walls and, preferably also, a catheter as it isbeing advanced through the blood vessel to a stenosed portion of theblood vessel. Using such a visual display facilitates use of ultrasoundangioplasty without the need for bypass surgery.

FIGS. 9 and 10 illustrate an ultrasound angioplasty device 128. Inparticular, FIG. 9 shows a schematic view of the device being used toclear a human artery 129. Walls 131 of the artery define a passageway133, which at a stenosed portion 135 of the artery seen in FIG. 9 isobstructed by arterial plaque 137. To remove the plaque 137 as part ofthe angioplasty procedure, the angioplasty device 128 makes use of anultrasound catheter 139, which receives ultrasound from an ultrasoundgenerator 141 located outside the patient's body. The ultrasoundgenerator 141 is preferably configured as described above, withreference to FIGS. 6 and 7, such that intense ultrasound waves areefficiently coupled to the ultrasonic catheter 139. Ultrasound producedby the generator 141 is conveyed by an acoustic waveguide 143 which iscomposed of a nickel-titanium material which is flexible and transmitsultrasound very well. Ultrasound waves are generated at a first end 145of the waveguide 143, as has been previously described, and is conveyedwithin the to an extendable, bulbous termination 147 of the catheter (ata second 148 of the waveguide). As alluded to earlier, the ultrasoundgenerator 141 preferably makes use of an end fire array of tenring-shaped shear wave transducers, mounted about a periphery of thefirst end 145 of the waveguide.

The ultrasound catheter 139 is shown in FIG. 10, and it includes anouter sheath 149 which houses the extendable termination 147 until thecatheter has been advanced to the stenosed portion 135 of the artery. Atthat point in time, a balloon device 151 of the sheath or equivalentmechanism is selectively used to lock the catheter in place with thewalls 131 of the artery, and the extendable member is then moved fromthe sheath toward the stenosed portion 135. The ultrasound generator 141may then be activated to cause the termination 147 to vibrate. Thecatheter 139 may be a triple lumen catheter, and may include additionaltubes which supply and extract fluid from the stenosed portion, for thepurpose of removing plaque splinters which are lifted from the arterywalls by the probe.

There are many ultrasound catheters which can be used as part of theultrasound angioplasty device 127 disclosed herein. Selection of asuitable ultrasound catheter is left to discretion of one or ordinaryskill, and examples of suitable catheter design may be observed, forexample, in U.S. Pat. Nos. 4,870,953, 5,209,719, 5,269,297 and5,304,115, and International Publication Number WO 92/11815 which arehereby incorporated by reference.

V. Application To A Resonant Measurement System

FIG. 11 shows an embodiment of the present invention which is used formeasurement of physical conditions, or alternatively, as a detector ofreflected ultrasound. In this resonant acoustic system 153, two phasedarrays are utilized, including one array 155 used as an acousticgenerator (such as illustrated by FIG. 6), and a second array 157 asacoustic detector (such as illustrated by FIG. 8). The system 153 doesnot directly use a source of electric power to generate ultrasound, butrather relies upon background noise and electronic amplification byamplifier 173 to create a resonant condition in a waveguide 159.

A first end 161 of the waveguide is closed, and helps reinforceproduction of longitudinal waves by the ultrasound generator, asindicated by the directional arrow 162. If the waveguide 159 is used tomeasure ambient physical conditions, for example pressure ortemperature, the waveguide is exposed to these conditions at a locationin-between the generator 155 and the detector 157, for example, bydirect exposure. An arrow 163 is used in FIG. 11 to indicate applicationof pressure to the waveguide 159, for example, for detecting pressurewithin a vacuum chamber. The pressure causes the waveguide to bend,thereby increasing or decreasing path length from ambient conditions,which correspondingly affects the phase of the acoustic wave detected bythe acoustic detector 155. The phase change causes a proportional changeof the resonant oscillation frequency. In this system, a second end 165of the waveguide proximate to the detector may be closed in a manner toconstructively reflect waves at the particular frequency the detector istuned to.

The acoustic detector 157 utilizes electric leads to provide an arrayoutput in the manner described above in connection with FIG. 8. Theindividual transducers generate electric output signals (indicated inFIG. 8 as either φ₁ or φ₂) that are retarded by an appropriate phase andthen summed together to generate an array output 167 of the detector'sphased array that collectively represents strength of detected acousticenergy. This array output 167 may then be processed and visuallydisplayed, such as by a meter or a display 169 seen in FIG. 11, inconnection with processing electronics 171. In addition, the arrayoutput 167 is also passed through a gain device 173 and used to generatean oscillation signal 175 that drives the acoustic generator 155. Inthis instance, the excitation source for the acoustic generator includesthe gain device 173 and the array output 167 provided by the acousticdetector 157. The oscillation signal 175 is provided to each of tentransducer rings of the acoustic generator 155 (with appropriate phaselags) to generate ultrasound and help create the resonant condition.

As an alternative, the resonant acoustic system 153 just described canalso be used to detect surfaces, such as specific textures or liquidlevel, for example. In this instance, the waveguide seen in FIG. 11 isnot terminated at the second end 165, but rather, directs acoustic wavesfrom an opening 177 and toward a target 179 that is to be measured.Acoustic waves are reflected back from the target to the waveguide (asindicated by arrow 181) and constructively or destructively combine withthe acoustic waves to change acoustic energy detected by the acousticdetector. The processing electronics 171 are appropriately configured toprovide the desired monitoring of measurement conditions to the user.

Those desiring additional information regarding the use of an ultrasoundsystem as just described can be obtained from the article "PhysicalSensors Using SAW Devices," by J. Fleming Dias, which appeared in theHewlett-Packard Journal, December 1981, which is hereby incorporated byreference.

VI. Fabrication Of The Transducers And End Fire Array

The fabrication of the transducers used in the end fire array will beexplained with reference to FIGS. 12-17.

Individual transducers are cut from a block 185 of piezoelectricmaterial (PZT), which may have a poling vector 187 as seen in FIG. 12. Adiamond core drill is utilized for this purpose, to core the PZT block185 and remove a center cylindrical section 189 from the block. As seenin FIG. 13, the cylindrical section 189 is then again cored along itsheight dimension, to form a bore 191 in the cylindrical section using aceramic lathe. The outer diameter of the cylindrical section is thenadjusted to match the appropriate design thickness for the transducerrings. Following that procedure, a diamond saw is then used to slice thecylindrical section 189 perpendicular to the height dimension to formindividual rings 193. These annular rings are parallel lapped to acommon thickness to prevent generation of spurious acoustic modes. Theindividual rings 193 may have an unknown poling vector at this point inthe process, which must be correctly set for the rings to correctlyoperate as shear wave transducers.

Accordingly, as seen in FIG. 14, each individual ring 193 isvacuum-coated with a conductive electrode (such as a gold-chromiummixture) 195 on either lateral side of the ring. The poling vector ofthe PZT sheet is reset by applying a very high voltage across theelectrodes 195, on the order of 60- to 80-volts per mil of thickness ofthe PZT ring. In the preferred embodiment, rings are cut to beapproximately 1/2λ_(L) in lateral (as opposed to radial) thickness. Oncethis step is performed, a new poling vector is created which isperpendicular to the geometry, as indicated by the reference arrow 197of FIG. 14. The electrodes 195 are then removed from the lateral facesof the ring 193 by use of a lapstone or an equivalent etching process toproduce a ring 193 that does not have any lateral electrode material, asindicated by FIG. 15.

Following electrode removal and resetting of the poling vector 197, newperipheral electrodes must be deposited on the radial surfaces 198 ofthe ring geometry to enable shear wave production upon application ofthe oscillation signal. Particle movement will be along the direction ofthe poling vector, with an oscillation signal motivating the rings tocreate sinusoidal particle motion and propagation of the shear waves.

As indicated in FIG. 16, deposition of the new electrodes is preferablyaccomplished by stacking the ring transducers 193 together and bysimultaneously vacuum-depositing the innermost and outermost electrodes199 and 200 to radial edges of the ring transducers. First, theinnermost electrodes 199 can be deposited using coated tungsten wires201 and 203, which are passed into a vacuum chamber 205 and through thebores of the ring transducers. The wires 201 and 203 are thensequentially heated to deposit layers of electrode material in anevaporation procedure. Preferably, a first one 201 of the tungsten wireshas been coated with chrome, and is used to apply a thin chrome layer207 to improve adhesion of a principal conductor layer 209, preferablygold. Prior to this procedure, lateral sides 211 of the ring transducersare deposited with a mask layer 213 so that no electrode material isdeposited on them. A second one of the tungsten wires 203 is preferablycoated with gold, and is heated to deposit the second, gold layer 209 tocomplete the electrode formation in the inner bore. Deposition of theoutermost electrode 200 is similarly performed, with the transducers 193rotated during the deposition procedure to promote uniform thickness inthe electrodes. Following electrode deposition, the mask layer 213 isremoved and the ring transducers 193 are ready for connection to thewaveguide.

FIG. 17 illustrates electrical and physical installation of each ringtransducer 193 upon a waveguide 217, and notably, the mask layer 213 hasbeen removed as indicated by phantom lines 219 of FIG. 17. Prior toinstallation, each transducer ring 193 and the waveguide 217 are cleanedin soap and scrubbed using a small brush. The waveguide and rings arethen rinsed in a series of ultrasonic baths, including sequential bathsof methanol, acetone, and methanol. In each case, duration of theultrasound bath is preferably at least 15 minutes. Each of theaforementioned parts are then dried in an oven and stored in dryconditions until the mounting procedure. For the mounting procedure,each transducer is coaxially fitted about the waveguide 217, such thatthe waveguide passes through the bore of all of the transducers. Theepoxy is a 2-part mixture of premixed epoxy which is stored a lowtemperature (-40 Fahrenheit).

In general, a bonding fixture (not shown) is used to simultaneouslymount all of the transducers and associated Teflon spacer rings 218. Theepoxy is applied to both of the waveguide 217 and the inner bore of eachtransducer 193, and the fixture is then used to simultaneously load allof the transducers and spacer rings. The entire waveguide assembly isthen put in an oven at 52 deg centigrade for a period of eight hours, toallow the epoxy to cure.

Electrical contact is made to each transducer 193 by connecting anelectronic lead 221 to the outermost electrode 201 of each transducer193, and by direct contact between each transducer's innermost electrode199 and a thin conductive electrode 223 deposited on the periphery ofthe waveguide 217. A single lead 225 may be used to connect the thinconductive electrode 223 of the waveguide to a transformer center tap,as with center taps 81 or 127 (seen in FIGS. 6 and 8, respectively).

As can be seen from the above, the present invention provides anacoustic system that efficiently couples sound with a waveguide, andgenerates highly directional, intense sound. The present inventionthereby provides utility to fields of measurement, medicine,communications, and other fields as well.

Various modifications of the exemplary embodiment described above willoccur to those having skill in the art. For example, differenttransducer spacings could be employed, with the transducers excited byelectrical phases of other than 180-degrees (e.g., a three-phase systemcould be implemented, using three electrical phases separated120-degree). Alternatively, different transducers within an array couldbe made to generate different frequencies of ultrasound. Further still,many different transducer poling arrangements could be used. Forexample, transducer poling in the end-fire array could be alternated, toeliminate the need for a push-pull excitation source.

Having thus described an exemplary embodiment of the invention, it willbe apparent that further alterations, modifications, and improvementswill also occur to those skilled in the art. Further, it will beapparent that the present invention is not limited to the specific formof a system for coupling acoustic energy described above. Suchalterations, modifications, and improvements, though not expresslydescribed or mentioned above, are nonetheless intended and implied to bewithin the spirit and scope of the invention. Accordingly, the foregoingdiscussion is intended to be illustrative only; the invention is limitedand defined only by the various following claims and equivalentsthereto.

I claim:
 1. An acoustic system, comprising:an acoustic waveguide havinga waveguide axis along which acoustic waves are capable of beinglongitudinally transmitted, and a waveguide periphery; and an acousticshear wave transducer positioned to occupy at least two differentpositions at the periphery, the shear wave transducer adapted totransduce shear waves propagating in a plane substantially perpendicularto the waveguide axis; wherein the at least two different positions areselected such that, when the transducer is driven, the transducergenerates shear waves which propagate toward the waveguide axis andconverge in mutual reinforcement, to thereby form a sweet spot withinthe acoustic waveguide, and the waveguide is effective to propagatecorresponding longitudinal waves along the waveguide axis.
 2. Anacoustic system according to claim 1, wherein the shear wave transducerforms a substantially continuous transducer which extends around thewaveguide periphery.
 3. An acoustic system according to claim 2, whereinthe acoustic waveguide has a substantially circular periphery incross-section, and the shear wave transducer is a ring transducerpositioned coaxial to the acoustic waveguide.
 4. An acoustic systemaccording to claim 1, wherein the shear wave transducer includes atleast two separate transducer segments that are driven by a commonoscillation signal.
 5. An acoustic system according to claim 1, whereinthe shear wave transducer includes at least two different pairs ofsegments, each pair of segments transducing shear waves of differentfrequency.
 6. An acoustic system according to claim 1, wherein:theacoustic system further comprises an excitation source that produces anelectronic oscillation signal, the electronic oscillation signaloperatively coupled to the shear wave transducer to drive the shear wavetransducer; and the transducer generates acoustic shear waves inresponse to the oscillation signal, with corresponding longitudinalwaves being propagated along the waveguide axis.
 7. An acoustic systemaccording to claim 6, wherein:the system is embodied in an ultrasoundangioplasty device, and the shear wave transducer is an ultrasoundtransducer; the acoustic waveguide has two ends, including a first endproximate to the phased array and a second end; and the ultrasoundangioplasty device includes an ultrasound catheter for invasive use in aliving body, the ultrasound catheter coupled to the second end toreceive ultrasound therefrom.
 8. An acoustic system according to claim1, wherein:the transducer is adapted to detect acoustic shear waves inresponse to longitudinal waves being propagated along the waveguide axisat the predetermined frequency; and the acoustic system furthercomprises an electronic output from the shear wave transducer which isproduced in response to acoustic waves detected by the shear wavetransducer, the output indicating strength of longitudinal waves at apredetermined frequency corresponding to the transducer.
 9. An acousticsystem according to claim 8, wherein:the acoustic waveguide includes afirst end and a second end, the transducer positioned at the second endof the acoustic waveguide; and the system further comprises an acousticgenerator capable of generating acoustic waves in response to anoscillation signal, the acoustic generator positioned at the first endof the acoustic waveguide, a feedback gain circuit adapted to receivethe electronic output and produces the oscillation signal in response tothe electronic output, and a dismay dependent upon the electronicoutput, the display thereby indicating change in the physical path thatlongitudinal waves travel along the waveguide axis.
 10. In an acousticdelivery system that includes an excitation source, an acousticgenerator that the excitation source causes to generate acoustic energy,and a waveguide that delivers acoustic waves from the generator to aremote location along a waveguide transmission axis, the improvementcomprising:at least two shear wave transducers of the generator,positioned at different points along the waveguide axis, each transducerhaving a shear wave transmission plane which is perpendicular to thewaveguide transmission axis, each transducer configured to generateshear waves of a predetermined frequency; wherein the different pointsof the shear wave transducers are selected to cause constructivereinforcement of the acoustic waves when the transducers are driven atthe predetermined frequency.
 11. An improvement according to claim 10,further comprising:a phase delay coupled between at least one transducerand the excitation source, to thereby cause relative delay in productionof shear waves between two transducers, the phase delay selected tocorrespond to a spatial interval between the two transducers to causethe two transducers to mutually reinforce propagation of a longitudinalwave along the transmission axis.
 12. An improvement according to claim10, wherein said improvement is embodied in an ultrasound angioplastysystem having a catheter-mounted bulbous termination that deliversvibrational energy to a stenosed region of a blood vessel, thetermination coupled to the waveguide at the remote location, theimprovement further comprising:using ultrasound transducers to produceultrasound shear waves; and utilizing a waveguide to couple theultrasound to the termination; wherein the termination responsivelydelivers vibrational energy to the stenosed region of the blood vessel.13. An improvement according to claim 10, wherein said improvementfurther comprises:using the waveguide to couple ultrasound to the remotelocation; and using a ring transducer for each shear wave transducer,each ring transducer having a bore which receives the waveguide in amanner such that the ring transducer fits around a periphery of thewaveguide.
 14. An improvement according to claim 13, wherein saidimprovement further comprises:using ring transducers that are circularlysymmetric about the waveguide.
 15. A method of transducing acousticenergy using a waveguide having a waveguide axis along which acousticwaves are longitudinally transmitted, a plurality of shear wavetransducers having an associated acoustic frequency, and electricalcouplings of the transducers, which carry electric signals correspondingto the particular acoustic frequency, comprising:positioning the shearwave transducers proximate to the waveguide and along it such that thetransducers transduce shear waves which propagate along a shear waveplane perpendicular to the waveguide axis; spacing the plurality oftransducers along the waveguide axis at fractions of a wavelength(corresponding to the particular acoustic frequency); and equalizingrelative phases of the plurality of transducers by providing phase lagsto them; wherein the shear wave transducers are spaced at intervalsrelative to the phase lags such that the shear wave transducerscollectively form a phased array tuned to the particular acousticfrequency, to thereby transduce the acoustic energy.
 16. A methodaccording to claim 15, wherein the waveguide includes an acousticwaveguide and the plurality includes at least five circularly-symmetricring transducers in parallel, spaced apart relation along the waveguideaxis around the periphery of the acoustic waveguide, furthercomprising:generating shear waves in a symmetric, radially-inward mannerwithin the acoustic waveguide, such that shear waves are maximized inamplitude substantially at a center axis of the acoustic waveguide, andare transmitted longitudinally substantially on the center axis.
 17. Amethod according to claim 15, wherein the ring transducers each includetwo pairs of transducer segments, each driven by different oscillationsignals, the method further comprising:providing each of the differentoscillation signals to a pair of segments; and generating at least twodifferent shear waves to concurrently propagate two independentlongitudinal waves along the waveguide axis.
 18. A method according toclaim 15, further comprising using the phased array as a sonic detectorand producing an electronic output representing magnitude of sound inthe waveguide at the particular acoustic frequency.
 19. A methodaccording to claim 18, further comprising applying gain to theelectronic output to form an amplified output, and applying theamplified output to an ultrasound generator to form a resonantultrasound system.